My invention relates to the speed control of electric motors, and more specifically to a method of, and a system for, driving an alternating current (AC) motor through a pulse width modulated inverter.
The technique of pulse width modulation (PWM) is well known by which an inverter operating from a fixed voltage, direct current (DC) supply can generate an AC output of variable frequency and voltage. Also known is the application of a PWM inverter for the speed control of an AC motor, as disclosed for example in the article entitled "New PWM Technique Using a Triangular Carrier Wave of Saturable Amplitude", by Tung Hai Chin et al., in the Vol. 1A-20, No. 3, May/June 1984 issue of IEEE Transactions on Industry Applications.
The PWM inverter for such motor speed control may have three pairs of switching devices such as transistors for driving a three phase motor. Conventionally, the three pairs of inverter switches have usually been activated independently by three different control signals. The phase relationship of the three phase control signals must be so critical that it has often been difficult to operate the inverter in a manner optimum for the desired motor speed control.
An obvious solution to this problem is the joint control of the inverter switches, as described and claimed in Asano et al. U.S. Pat. No. 4,477,763, dated Oct. 16, 1984. This patent teaches the driving of a three phase AC motor or like rotary machine by six different voltage vectors for creating a rotary magnetic flux in the motor, and a zero vector for arresting the rotation of the magnetic flux, the voltage vectors and zero vector being both determined by prescribed on off patterns of the inverter switches. The on off patterns of the inverter switches for the production of the desired voltage vectors and zero vector may previously be written on a memory or storage. This memory may then be read for activating the inverter switches in real time. Alternatively, the required inverter control signals may be generated without use of a memory, and their frequencies may be varied for correspondingly changing the output frequency of the inverter.
Either way, the output frequency of the inverter has been variable only by changing the frequency of the clock signal used as a time base for the inverter control signals. Changing the frequency of the clock signal is itself an easy task: A variable frequency source such as a voltage controlled oscillator (VCO) may be employed as a source of the clock signal, its frequency being variable by changing the applied voltage. However, not only is such a variable frequency source significantly more expensive than a fixed frequency source, but further its response is not so quick as can be desired for accurate motor speed control. This latter drawback manifests itself as an even more serious problem when the motor is being driven at ultralow speed.